1. Field of the Invention
The present invention relates to a projection optical system and a projection display device, and more particularly, to a projection optical system that projects an image displayed on an image display device onto a screen using a first optical system including a plurality of lenses and a second optical system including a concave mirror and a projection display device using the same.
2. Description of the Related Art
In general, in projection optical systems used in projectors, the back focal length needs to be long, or an entrance pupil as viewed from the reduction side (light valve side) needs to be disposed at a sufficiently distant point, that is, the reduction side needs to have telecentricity.
With an improvement in the performance of light valves, it is necessary to effectively correct aberrations according to the resolution of the light valve. In addition, there is a strong demand for a fast and wide-angle projection optical system for presentation considering use in, for example, a relatively bright and small indoor space.
A projection optical system has been proposed which includes a first optical system, which is a refracting system including a plurality of lenses, and a second optical system, which is a reflecting system including a concave mirror (see JP-A-2004-258620, JP-A-2006-235516, and JP-A-2007-79524). In this type of projection optical system, a relay system is provided between the first optical system, which is a refracting system, and the second optical system, which is a reflecting system. Therefore, it is possible to reduce the focal length and the projection optical system is suitable for increasing the angle of view.
However, JP-A-2004-258620 discloses a complicated eccentric optical system in which a plurality of optical surfaces is shifted or tilted relative to the optical axis. Therefore, it is very difficult to assemble the eccentric optical system.
JP-A-2006-235516 and JP-A-2007-79524 disclose a coaxial optical system. Therefore, it is easy to assemble the coaxial optical system. However, the coaxial optical system is mainly configured for a rear-projection-type projector. Therefore, the coaxial optical system is not suitable to be provided in a front-projection-type projector since the overall size of the system is large.
In order to obtain a projection optical system that is sufficiently small to be provided in a front-projection-type projector, it is necessary to reduce the total length of the optical system and the size of the concave mirror. However, in this case, it is expected that various aberrations, such as distortion, will occur. Therefore, an important technique problem is to take measures to reduce the aberrations.